1. Field of the Invention
The present invention relates to an image forming apparatus having an image heating apparatus that is adapted to hold and convey a recording material that bears an image in a nip between a heating member and a pressurizing member, to at least one of which a bias is applied, so as to heat the image on the recording material.
2. Related Background Art
Conventionally, a heat fixing apparatus using a heating roller scheme and a heat fixing apparatus using a film heating scheme or the like have been adopted as heat fixing means serving as an image heating apparatus to be equipped in an image forming apparatus such as a copying machine, a printer, a facsimile machine or the like that uses, for example, an electrophotography process. The heat fixing apparatus using the heating roller scheme is of a contact heating type that has a good heat efficiency and a high degree of safety, while the heat fixing apparatus using the film heating scheme has a quick-start ability (or an on-demand ability) and it is an energy-saving apparatus that is not supplied with electric power during a stand-by state so that power consumption would be reduced as little as possible.
The heat fixing apparatus using the heating roller scheme has a rotating fixing roller serving as a heating member that is heated by a built-in heat source such as a halogen heater and controlled to a predetermined temperature and a rotating pressure roller serving as a pressurizing member that is in pressure contact with the fixing roller at a predetermined pressure, and a recording material is introduced into a pressure contact nip portion (or a fixing nip portion) between those rollers so as to be held and conveyed therein, so that an unfixed image is heated to be fixed on the surface of the recording material.
The heat fixing apparatus using the film heating scheme is disclosed in prior art documents such as Japanese Patent Application Laid-Open No. 63-313182 and Japanese Patent Application Laid-Open No. 1-263679. The heat fixing apparatus using the film heating scheme causes a heat resisting film to run as a moving member under the state in which it is closely pressed to a heating member by a pressurizing member and introduces a recording material between the heat resisting film and the pressurizing member in a fixing nip portion formed by the heating member and the pressuring member with the heat resisting film between to bring the recording material into close contact with the heat resisting film so as to cause the recording material to pass through the fixing nip portion together with the heat resisting film. Thus, heat of the heating member is imparted to the recording material via the heat resisting film, so that an unfixed image is heated to be fixed on the surface of the recording material.
FIG. 7 shows the outline of an example of the heat fixing apparatus using the film heating scheme. In FIG. 7, reference numeral 27 designates a fixing unit serving as a heating member and reference numeral 18 designates an elastic pressure roller serving as a pressurizing member. These members 27 and 18 are in pressure contact with each other to form a fixing nip portion N.
The fixing unit 27 serving as the heating member is composed of a stay holder 17, a heating member 15 such as a ceramic heater securely held on the bottom surface of the stay holder 17 and a cylindrical fixing film 14 functioning as an elastic moving member loosely fitted on the stay holder 17. The fixing film 14 includes, in the following order from the outer surface, an insulative releasing layer, an electrically conductive layer and a substrate layer.
The elastic pressure roller 18 serving as the pressurizing member includes a metal core 29, an elastic layer 30 and a surficial insulative releasing layer 31.
The heating member 15 in the fixing unit 27 and the elastic pressure roller 18 serving as the pressurizing member that are mentioned above are opposed to each other, so that the fixing film 14 is held in a pressure contact and the fixing nip portion N is formed.
The elastic pressure roller 18 is driven to rotate counterclockwise. The fixing film 14 follows the rotation of the elastic pressure roller 18, and the fixing film 14 is brought into a clockwise rotating state as shown by an arrow while its inner surface is in close contact with the heating member 15 and sliding thereon in the fixing nip portion N.
A recording material P on which an unfixed toner image T is formed and born is introduced into the fixing nip portion N between the fixing film 14 and the elastic pressure roller 18 so as to be held and conveyed. In this holding and conveying process, the unfixed toner image T on the recording material P is heated in the fixing nip portion N by the heating member 15 via the fixing film 14 so as to be fixed with a pressurizing force of the fixing nip portion N applied thereto.
Both the heating roller scheme and the film heating scheme suffer from the following problems with respect to image quality that arise in the fixing process.
(1) Just before the introducing portion of the fixing nip portion N, a part of the unfixed toner image T on the recording material P is blown away in the downstream side of the recording material conveying direction. (This phenomenon will be referred to as “backward toner flying” hereinafter.)
(2) A part of the unfixed toner image T on the recording material P is not fixed but adhering to the fixing film 14 or the fixing roller, so that when the portion of the fixing film 14 or the fixing roller at which the toner is adhering is subsequently in contact with the recording material P, the toner on the fixing film 14 or the fixing roller is transferred to the recording material P to cause an image error. (This phenomenon will be referred to as “offset” hereinafter.)
In order to suppress these phenomena, methods for enhancing electrostatic adhering force of the toner to the recording material P have been conventionally developed.
Referring, for example, to the above-described heat fixing apparatus using the film heating scheme shown in FIG. 7, there has been adopted a method in which a predetermined amount of fixing bias having the same polarity as the unfixed toner image T (in this example the charge polarity of the unfixed toner image T is minus) is applied to the electrically conductive layer of the fixing film 14 (which includes an insulative releasing layer, an electrically conductive layer and a substrate layer in the mentioned order from the outer surface) by a fixing bias applying power source E via an electrically conductive brush 51 to press down the unfixed toner image T on the recording material P on the recording surface by means of a repulsive electric field with the surficial insulative releasing layer 31 of the elastic pressure roller 18 between. Alternatively, a method in which a fixing bias having the polarity reverse to the unfixed toner image T is applied to the metal core 29 of the elastic pressure roller 18 to induce a charge with the polarity reverse to the unfixed toner image T on the surface of the elastic pressure roller 18 so as to attract the unfixed toner image T toward the recording material P has also been adopted.
In order to enhance the effect further, a method as shown in FIG. 8 has been adopted. In this method, a static elimination means 52 such as an electrically conductive brush is provided downstream of the fixing nip portion N with respect to the recording material conveying direction in such a way that the static elimination means 52 is in contact with the surface opposite to the recording surface of the recording material P that has passed through the fixing nip portion N, and a bias having a polarity the same as or reverse to the unfixed toner image T is applied to the fixing film 14 or the elastic pressure roller 18 in a similar manner as the above-mentioned methods. Thus, a charge having a polarity the same as or reverse to the fixing bias applied from a mount portion of the static elimination means 52 is induced via the resistance of the recording material P, so that the induced charge attracts the unfixed toner image T that has the reverse polarity to the recording material so as to fix the unfixed toner image T thereon.
There is an optimum value for the fixing bias for preventing the electrostatic offset, and a fixing bias larger than or smaller than the optimum value would make electrostatic offset worse. On the other hand, it is desirable that the fixing bias for preventing the backward toner flying is as high as possible. Therefore, if the value of the fixing bias is set to the optimum value for the electrostatic offset, the backward toner flying will be significantly generated, though the electrostatic offset can be prevented completely. On the other hand, if the fixing bias is set as high as possible in order to prevent the backward toner flying completely, the electrostatic offset will be significantly generated. In view of the above situations, the fixing bias is set to a value with which both the electrostatic offset and the backward toner flying are kept to satisfactory degrees.
In the above-described heat fixing apparatus, in the case that recording materials (or small size media) having a width smaller than the maximum passable sheet width of the apparatus are consecutively fed and the heat fixing process is ceaselessly performed, the temperature of such an area (which is referred to as a non-sheet passing area) in the fixing nip portion through which the recording materials do not pass will continuously increase, since there is no media that removes heat from that area. On the other hand, the temperature of such an area (referred to as a sheet passing area) in the fixing nip portion through which the recording materials pass is kept to a predetermined temperature by a temperature control system. Therefore, the temperature difference between the non-sheet passing area and the sheet passing area of the fixing nip becomes large, which is a so-called over temperature rise phenomena in the non-sheet passing portion.
In the graph shown in FIG. 9, the abscissa axis represents time. The area designated with the caption “SMALL SIZE PRINT” represents the period through which printing was consecitively performed on small size sheets. After that period, printing was consecutively performed on standard size sheets, which corresponds to the area designated with the caption “STANDARD SIZE PRINT”. The temperature of the non-sheet passing area and the sheet passing area was measured, and the graph illustrates that the temperature rise in the non-sheet passing area becomes large.
When printing is performed on a recording material (a large size media) having a width larger than the above-mentioned recording material while the over temperature rise phenomena is occurring in the non-sheet passing area, the temperature distribution in the longitudinal direction within the fixing nip portion is like shown in FIG. 10. Specifically, since in the sheet passing area with respect to the small size media a predetermined fixing temperature has been kept, a satisfactory fixed image can be obtained in the image area corresponding to this sheet passing area with respect to the small size media. However, in the non-sheet passing area temperature has been raised due to the printing on the recording material with a small width, namely the temperature has been raised higher than the predetermined temperature for the sheet passing area. Therefore hot offset due to over-fixing will occur in the image area corresponding to this non-sheet passing area with respect to the small size media.
In addition, under such a state in which the temperature in the fixing nip portion is very high and hot offset can easily occur, the process is very sensitive to the fixing bias, and therefore if the fixing bias is not appropriate, a severe image error that involves both the electrostatic offset and the hot offset will occur.
In order to prevent this hot offset, when printing is to be performed on an standard size recording material(s) after the printing on small size recording materials, the fixing temperature is reduced to a temperature lower than the normal fixing temperature for standard size recording materials in accordance with the number of the small size recording materials that have been printed as shown in Table 1 so as to suppress the hot offset.
TABLE 1Number of Printed Small Size1-1011-3031-5051-SheetsTemperature Setting for Standard−10° C.−20° C.−30° C.−40° C.Sheets
However, as the number of the printed small size recording materials increases, the temperature rise in the non-sheet passing area becomes so high that a certain degree of hot offset would occur, even if the fixing temperature for standard recording materials is reduced as shown in Table 1. In addition, if the fixing temperature is further reduced with the view of preventing the hot offset, a fixing error will occur in the sheet passing area with respect to the small size recording materials.